1. Field of the Invention
The present invention relates to an electro-optical device such as a liquid crystal display device or a similar type of display device and to a method of driving the same. In particular, the present invention relates to a display device operating with an active matrix, a method of displaying images with said device, and a process for fabricating said device.
2. Prior Art
With the increasing demand for various types of more compact and energy-conserving equipments for office automation (OA), the conventional display devices using cathode ray tubes (CRTs) are being replaced by flat panel displays (FPDs) such as liquid crystal displays (LCDs) and plasma display panels (PDPs). Particularly among them, the LCDs are advantageous in that they are least power consuming, and hence are most preferred for use in portable equipments.
However, LCDs have yet numerous problems to be solved. Most of the LCDs which are used in practice at present are those operating in a single matrix, and are sometimes called by the name of the liquid crystal material used therein as STNLCDs (super-twisted nematic LCDs). The STNLCDs have prevailed because they can be fabricated by a simple process and, therefore, at a low cost.
The super-twisted nematic liquid crystals are, however, extremely sluggish by nature, and its poor response fails to display an object moving at a high speed.
Concerning its mode of operation, a pixel remains "on" for a duration of from several tens of microseconds (.mu.sec) to 1 millisecond (msec) per 1 frame (in general, a frame has a duration of from 10 to 30 msec). The duration of the pixel in its "on" state is related to the inverse proportion of the number of lines. Thus, in a matrix consisting of 200 lines, it can be calculated, by assuming 30 msec per frame, that the pixel is "on" for about a mere 150 .mu.sec. This results in a picture plane of low contrast, and, moreover, in a picture plane which is not visible from an oblique angle. Furthermore, when an extremely bright or dark portion is included in the picture plane, such a portion affects the surroundings; i.e., there arises a problem of "cross-talk".
Recently, the types of LCDs operating on a mode different from the aforementioned single-matrix mode are being proposed and are made commercially available. Those are called collectively as LCDs operating on an active matrix. The pixels in this type of LCDs are each switched individually by their respective active elements which are connected thereto. Those LCDs can be further classified into TFTLCDs and MIMLCDs, according to the type of the active matrix used therein. TFT is an abbreviation for thin film transistor, and MIM stands for a diode having a metal/insulator/metal structure.
The LCDs operating on an active matrix mode has a high contrast, and the visual angle thereof is wide. These are ascribed to the fact that the pixel in those LCDs remains "on" for a duration almost equal to that of the frame. However, those LCDs have technological problems in their fabrication, and hence, the product yield is still too low. Thus, because of high cost and of its being too expensive to purchase, their practical application at present is only limited to the displays of elaborate computers.
It should be noted that, despite the demand for LCDs are now limited principally to portable computers, the market for those LCDs are expected to expand in near future because there would be demands in a wide application field, such as cordless telephones, displays attached to portable telephones, and information displays of electronic dictionaries and the like. Furthermore, apparatuses such as a one equipped with a display having a large image plane for viewing newspapers and other printed matter are also prospects which may establish a large market.
The most important point in making displays feasible in the applications described hereinbefore is not a high quality display having a gradation, but how to conserve energy. However, none of the conventional LCDs were satisfactory on this point, because any of the STNLCDs, TFTLCDs, and MIMLCDs must at least rewrite the image at least once per i frame.
Furthermore, the displays intended for the aforementioned special use are generally used continuously over a long time, displaying the same image. Thus, the power source must be turned off during the idle time of the display to save power. However, on the other hand, it is loss of power transferring image from the memory each time the display is off, and, moreover, it is time consuming to read out the image each time. Accordingly, it is desirable that the display itself has a memory function so that the image may be restored to the display upon its turning on.
A low cost display for this special purpose can be realized by the use of an STNLCD of a conventional type. However, the display would result in a low contrast and visually unfavorable due to its narrow visual angle. Furthermore, the display consumes much energy during the display is on, because a high voltage pulse of 20V or higher is flown to and fro at a frequency as high as 15 kHz.
A TFTLCD may be used in the display above to provide a favorable contrast and visual angle. However, it requires a high cost, and though the pulse voltage may be lowered to 10V or lower, the power consumption is still high.
A ferroelectric liquid crystal (FLC) may be used if a particular emphasis were to be posed on its memory functions. However, an FLC has one problem; even when the image is turned off, the image in the display remains unerased when too long a same image is displayed. Furthermore, the temperature range for an effective operation of the FLC is limited to a very narrow range.
In the applications of displays to personal computers, there are cases in which high response is not required. For example, the image in a word processor does not change so frequently as in the image of a television set. Only a part changes within a period of 1 second. However, in the conventional LCDs, it happens, unnecessarily enough, that the whole image is written and erased as frequently as 30 times per second. In doing so, the signals of the portions which need not be rewritten are processed and transferred. Accordingly, a large load is posed on the function of a display.
Furthermore, in the digital-type displays for images having gradation (sometimes referred to hereinafter simply as "displays operating in a digital gradation mode") filed by the present inventors, an extremely large amount of signals must be processed. These are disclosed in Japanese patent application Nos. Hei 3-157504, 3-157503, 3-157502, 3-157505, 3-157506, 3-157507, 3-163870, 3-163871, 3-163872, 3-163873, 3-169306, 3-169307, 3-209869, and 3-209870. A part of those signals is used not for changing the image, but for maintaining a certain state. It can be seen, accordingly, that an LCD of a conventional type or of a modification thereof would consume a considerable amount of power on its operation at a high speed.
In the case of displaying a high speed motion of only a part of the image, such as displaying a cursor moving at a high speed on the display of a computer, it is required that the frame frequency is increased to not less than the conventional 30 Hz. However, in a conventional LCD, the whole image, inclusive of the static image, should be rewritten according to the frame frequency. Then, it happens that the signal processing function falls behind the movement of the image.
In FIG. 2 is shown schematically the pixel circuit of a conventional TFTLCD using a TN liquid crystal as the liquid crystal material. An example of its mode of function is also shown in the same figure. The TFT and the drain are each connected to a selection line (gate line) and a data line (drain line), respectively, and the source is connected to the pixel electrode. The counter electrode of the pixel electrode is a common electrode which is usually maintained at a constant voltage; in general, it is earthed.
A pulse is periodically applied to the selection line and the information of the pixel is applied to the data line as a voltage signal. This is shown in FIG. 2(B). The period of the pulse which is applied to the selection line corresponds to the period of a frame in the usual operation, and it is typically in the range of from 10 to 30 msec. The width of the pulse is generally about the length of the period being divided by the number of the lines or smaller. In the case of an information display and the like having a relatively small matrix of about 100 lines, the pulse duration is generally in the range of from 100 to 300 .mu.sec.
The signal is applied to the data line by bringing it into a high voltage state to express the "on" state of the pixel, and applying no voltage to express the "off" state of the pixel. The polarity of the high voltage is periodically changed, i.e., an alternating current is applied. Such a measure is taken, because a TN liquid material undergoes decomposition and degrades thereby if a direct current is applied to the TN liquid crystal material for too a long time.
The source side of a TFT to which a signal above is applied yields a signal V.sub.1, as shown in FIG. 2(B). At the initial state, the TFT turns "on" upon receiving a pulse from the selection line, and hence the source voltage increases to reach the drain voltage. However, upon the disappearance of the pulse, a voltage drop of 1/4V occurs on the source voltage due to the stray capacitance between the gate electrode of the TFT and the source region. Then, as the TFT turns "off", the pixel electrode becomes electrically isolated and the voltage thereof gradually decreases due to the leak current of the TFT.
Then, as the TFT turns "on" again upon application of a pulse, the source voltage this time approaches to a negative drain voltage. Upon cutting off of the pulse, the voltage shifts to the negative side this time by 1/4V due to the stray capacitance as in the case above, and the voltage again is attenuated due to the presence of leak current. Because the drain voltage is zero upon application of the final pulse of the selection line to the TFT, the charge having stored in the pixel electrode is discharged until V.sub.1 attains zero voltage.
As described in the foregoing, a TFTLCD basically operates in this manner. However, it is next to impossible to make all the pixels follow the same process in a uniform manner. First of all, the TFT must respond in a short period of time in the range of from 10 to 30 .mu.sec, and a little difference in the TFT characteristics considerably affects the output source voltage. In a TFT based on amorphous silicon having a small carrier mobility, in particular, the pulse applied to the selection line in such a short period of time is cut off before the pixel electrode is Sufficiently charged.
The amount of voltage drop .DELTA.V can be related to the stray capacitance C', the pixel capacitance C, and the amplitude of the selection pulse V.sub.G by an equation as follows EQU .DELTA.V=C'V.sub.G /(C+C').
It can be seen from this relation that the 1/4V increases with increasing stray capacitance; hence, as a result, the V.sub.1 as a whole shifts to a negative side (or a positive side) with increasing stray capacitance. This leads to the application of a direct current to the liquid crystal. Accordingly, a degradation occurs on the liquid crystal. Moreover, in the case where the frame frequency is about 30 Hz, the image suffers flickers; i.e., the image is alternately turned bright and dark every period of 15 Hz. The most serious problem here is that this 1/4V is not uniform for all the TFTs, and this large difference in 1/4V between the TFTs considerably reduces the product yield.
If an attempt were to be taken to increase the pulse width of the selection line to solve the aforementioned problems, the frame frequency should be lowered. This can be realized with no particular problem if the display is not intended for moving images. However, the TFT driven at too low a frequency signifies that it is driven substantially with a direct current, and again there is fear of causing damage to the liquid crystal. Furthermore, the charge having accumulated in the pixel electrode undergoes a spontaneous discharge. Accordingly, in practice, the frame period cannot be taken longer than, for example, 1 second.
Ideally, a TFT having fabricated from polycrystalline silicon (polysilicon) in a self-aligned manner is preferred for use because it has a high operation speed and a small stray capacitance. However, the fabrication of such TFTs requires thermal annealing at 600.degree. C. for a long period of time or the use of a special technique such as laser annealing and electron beam annealing. The use of a thermal annealing signifies that the substrate material must be selected beforehand so that the substrate material may endure the heat, and that a material for the metal wiring must be selected from those other than aluminum. An aluminum wiring indeed is most preferred for a metal wiring, but it would be seriously damaged at a temperature as high as 600.degree. C. Thus, the gate electrode must be made from a material selected from materials other than aluminum. Furthermore, thermal annealing requires heating for a duration of 24 hours or even longer. This is also a big problem. Moreover, the thermal annealing at about 600.degree. C. should be followed by a second thermal annealing in a high temperature, as high as in the range of from 900.degree. to 1100.degree. C. This step hence confines the substrate material to quartz. The use of quartz as the substrate material not only limits the area of the display, but also increases the production cost.
In contrast to thermal annealing, laser annealing and electron beam annealing processes are basically low temperature processes and hence are applicable to almost any kind of substrate material. However, both technologies are premature and await a still some time to obtain TFTs with high reliability. Furthermore, they are not suited for mass production.
In realizing the digital gradation according to the previous invention of the present inventors using the circuit shown in FIG. 2, a still faster operation is requisite. More specifically, for example, a digital gradation display with 18 gradation requires an operation speed as fast as 18 times the conventional speed. In this context, it is now believed that no TFT other than a self-aligned type polysilicon TFT is applicable in realizing such a high speed. However, this digital degradation display includes useless operations. In the digital gradation display system, the length of time during the voltage is applied to the pixel is divided, and then the length of each of the divided time segments is controlled so that the effective voltage applied to a pixel is thereby controlled by the duration of applying the voltage. The basic point resides, however, in that the frame frequency is increased as compared with a conventional one. However, most of the operation is occupied by, as in any other conventional LCD display methods, the unnecessary rewriting operation. This as a result consumes large amount of power.